Light emitting diodes (LEDs) are increasingly used in modern display technologies. LEDs have many advantages over traditional light sources including: low energy consumption, long lifetime, robustness, small size and fast switching. LEDs remain relatively expensive and require precise current and heat management relative to traditional light sources. Fabrication costs are significant and exceed the material costs for some LEDs. Conventional LEDs are made from inorganic compound semiconductors, typically AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue), which emit monochromatic light of a frequency corresponding to the band gap of the semiconductor compound used. These conventional LEDs do not emit light of mixed colors, for example white light. White LEDs can be used as light sources and are capable of producing full color displays with existing color filter technology. One method being used to produce white light is to combine individual LEDs to simultaneously emit the three primary colors, which mix to produce white light. Another method is to use a yellow phosphor to convert monochromatic blue light, or two or more phosphors emitting different colors to convert UV light, from a LED to broad-spectrum white light, although color control is limited by this approach. Organic LEDs (OLEDs) can also be fabricated relatively inexpensively to provide a variety of colors and white light, but OLEDs generally suffer from deficiencies in efficiency and in lifetime relative to inorganic devices as the light-emitting layer, being that of an organic material, typically requires a relatively high current density and driving voltage to achieve high luminance which promotes degradation of the OLEDs, especially in the presence of oxygen, water and UV photons.
Quantum dot light emitting diodes (QD-LEDs) are being developed for display and lighting sources. Inorganic quantum dot light emitters have a few advantages over OLEDs and other light-emitting diodes, which include stability, solution processability and excellent color purity. Quantum dots (QDs) are semiconductor nanocrystallites whose radii are smaller than the bulk exciton Bohr radius. Quantum confinement of electrons and holes in all three dimensions leads to an increase in the effective band gap of the QDs with decreasing crystallite size, where the optical absorption and emission of quantum dots shift to higher energies (blue shift) as the size of the dots decreases. For example, a CdSe QD can emit light in any monochromatic visible color depending only on the size of the QD and can be used to form QD-LEDs arrays that emit white light.
Current QD-LEDs employ a few layers of organic materials and reactive metals for efficient charge transport and injection. The use of the organics offsets some advantages of QD-LEDs and has discouraged commercialization of QD-LEDs. For instance, Sun et al., Nature Photonics, 2007, 1, 717 discloses tris(8-hydroxquinoline) aluminum (Alq3) as an electron transporting layer and calcium as an electron injection layer to obtain an efficient color QD-LED. Unfortunately, long term stability is insufficient due to degradation of the organic layer and oxidation of the reactive metal. Device fabrication requires a costly vacuum deposition method. For most QD-LEDs, defects can occur at the organic-inorganic interface between the QD-emitting layer and an organic electron transport layer, which lead to poor electron injection into the QD-emitting layer. Caruge et al., Nature Photonics, 2008, 2, 247 discloses a fully inorganic QD-LED with good long term stability. However, the charge transporting layers are fabricated through complicated and costly vacuum sputter deposition methods.
Cho et al., U.S. Patent Application Publication 20090039764 discloses a QD-LED where a continuous inorganic thin film is used to constitute the electron transport layer instead of an organic thin film. The inorganic thin film can be prepared by a cost effective solution coating process such as spin coating, printing, casting and spraying, followed by a chemical reaction, a sol-gel process, to form the inorganic thin film material after deposition on the light emitting layer. Preparation of all inorganic QD-LEDs or at least a device with an inorganic electron transport layer that does not require expensive processing steps or formation of a layer by a reaction process is desirable.